The present invention generally relates to a control system for a machine and, more particularly, to a control system for a operating machine of a type that is program-controlled by a microcomputer or the like.
When it comes to the sequence control of the operation of a machine by the use of a programmed control device such as a microcomputer, a control signal derived from the programmed control device must be synchronized with the actual operation of the machine having variables including the voltage, the load torque and the like.
The Japanese Laid-open Patent Publication No. 54-141134, laid open to public inspection on Nov. 2, 1979 and corresponding to the U.S. Pat. No. 4,280,763, discloses a control system effective to achieve the requisite synchronization. Briefly speaking, the control system of this publication comprises, as shown in FIG. 1 of the accompanying drawings, a rotary disc 4 mounted on a rotary shaft 2' of a drive system of the machine including a motor 1 for rotation together therewith and having a plurality of slits 3 defined therein, a light emitting element 5 and a photo sensor 6 so positioned on one side of the rotary disc 4 opposite to the light emitting element 5 that, during the rotation of the rotary disc 4 in synchronism with the motor 1, a beam of light travelling from the element 5 towards the sensor 6 can be cyclically intercepted with the sensor 6 consequently generating a train of pulses of a frequency equal to the cycle of interception of the beam of light passing through the slits 3, which is an indication of the speed of rotation of the motor 1. According to this prior art control system, the minimum possible value tl of the pulse cycle t (which corresponds to the maximum possible speed of rotation of the motor) is so selected to be greater than the time required for the programmed control device to complete its execution of one routine so that, after the pulse signal has been supplied from the photo sensor 6, the programmed processing routine can be looped or actuated.
In this prior art control system, so long as the machine is operated by the motor 1, the programmed processing routine for the programmed control apparatus can be maintained exactly synchronized with the actual operation of the machine even though the driving speed of the motor fluctuates. On the other hand, when and so long as the motor 1 is held in a standstill position, that is, when and so long as the operation of the machine need not be synchronized with the control signal derived from the programmed control device, the programmed processing routine can be looped in response to or immediately after the time-up of an internal timer (a digital timer for counting the number of reference clock pulses) provided in the programmed control device for defining the time during which the above described programmed processing routine must be completed.
Apart from the above, a system effective to check if the motor 1 is normally operated by the utilization of the feature that the pulse signal is generated in synchronism with the drive of the motor 1 is disclosed in the Japanese Laid-open Patent Publication No. 55-90967, of the same applicant, laid open to public inspection on July 10, 1980 and corresponding to the U.S. Pat. No. 4,269,500. Briefly speaking, the system disclosed therein is such that, after the execution of the process necessary to rotate the motor, a predetermined timer for the pulse checking is set to operate and in the event that no pulse signal is detected within a predetermined time set to the timer, the motor is determined as rotating in an abnormal condition.
In the control system referred to above wherein the pulse signal synchronized with the rotation of the motor (which pulse signal is hereinafter referred to as a motor pulse) is utilized, since the speed of rotation of the motor during any one of the starting or start-up period and the stopping or run-down period of the motor is different from that during the normal running period thereof, the routine is executed only by the internal timer built in the programmed control device during the start-up and run-down periods of the motor without utilizing the pulse signal even though such pulse signal has been generated upon rotation of the motor, as is the case when the motor is held in a standstill position. More specifically, an arrangement has been made such that, when a start signal has started the rotation of the motor, a timer having a preset time equal to the time apparently required for the motor to pass over the start-up period is activated, and the above described motor pulse is associated with the programmed control immediately after the preset time of the timer is passed over and, when an off signal for inactivating the motor is generated, the internal timer supersedes such timer.
However, since the speed of rotation of the motor tends to be largely affected by the load torque and the voltage, a delay time for the synchronization especially during the start-up period of the motor must be long enough to account for a safety purpose and, therefore, it requires a long time for a predetermined operation to be initiated. If the motor pulse detecting timing is accelerated in order to obviate the above described drawback, another disadvantage occurs in that, even though the motor has been rotated, the motor would be determined as operating in an abnormal condition if the starting of the motor is somewhat irregular.